feat: add Mimir (External Endpoint) Prometheus provider

[ni-mi]

Summary

Adds support for Grafana Mimir as a Prometheus metrics provider. Mimir is a Prometheus-compatible remote metrics backend that typically runs outside the cluster, so it cannot be accessed through the Kubernetes API service proxy like traditional in-cluster Prometheus deployments.

• New "Mimir (External Endpoint)" provider — queries metrics directly from an external Mimir URL, bypassing the K8s API service proxy
• Custom HTTP headers — supports X-Scope-OrgID (required by multi-tenant Mimir) and arbitrary key-value headers on every metrics request
• Dedicated settings UI — endpoint URL field, "Anonymous tenant" checkbox, and a dynamic header editor; hides irrelevant service-address/HTTPS fields

Motivation

Organizations using Mimir as a centralized metrics backend don't have a Prometheus service running inside each cluster. The existing Freelens Prometheus integration mandates a <namespace>/<service>:<port> address that gets proxied through the Kubernetes API. This PR adds a first-class option for reaching Mimir (or any external Prometheus-compatible API) directly.

Changes

File	What changed
packages/technical-features/prometheus/src/mimir-provider.injectable.ts	New provider using Helm-compatible PromQL queries with a synthetic (empty) service address
packages/technical-features/prometheus/src/register-injectables.ts	Register the new provider in DI
packages/core/src/common/cluster-types.ts	Add customHeaders field to ClusterPrometheusPreferences
packages/core/src/main/get-metrics.injectable.ts	Apply custom headers on directUrl requests; improve error messages with response body
packages/core/src/main/cluster/prometheus-handler/prometheus-handler.ts	Clarifying comment for directUrl-only providers
packages/core/src/renderer/components/cluster-settings/prometheus-setting.tsx	Mimir UI section (URL, anonymous tenant, additional headers); trim whitespace in path prefix

Test plan

• Select "Mimir (External Endpoint)" as Prometheus provider in cluster settings
• Configure endpoint URL and /prometheus custom path prefix
• Enable "Anonymous tenant" checkbox → verify X-Scope-OrgID: anonymous header is sent
• Add custom headers → verify they appear in outgoing requests
• Confirm metrics load successfully on Cluster Overview and Pod detail views
• Verify existing providers (Helm, Operator, OpenShift, etc.) are unaffected
• Verify settings persist after closing and reopening cluster settings

[ni-mi]

@robertobandini - I am not sure what is the process to get this PR approved or reviewed - I'd appreciate your help with it.
Thanks!